The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Polymeric foamed elements are traditionally made by placing polymeric beads within a mold cavity and passing steam through the mold cavity to melt the beads together to form the element. This process is commonly referred to as steam chest molding. Steam chest molding has several drawbacks. For example, the foam bead material is expensive, thereby increasing the finished part cost. Due to the amount of time required to first melt all of the bead material and subsequently to cool both the foam material and the mold, mold cycle time is long, up to about ten minutes or longer. Lengthy mold cycle time further increases the per part cost and decreases production efficiency.
A process wherein liquid polymer is poured into a mold is also known to form foamed material. This process involves mixing two liquefied component parts, typically a base polymer and a catalyst. The liquid foam mixture is poured into a mold and the part is allowed to solidify before removal from the mold. A chemical reaction occurs when the two component parts are mixed, resulting in expansion and hardening of the material. This process is suitable for use in open, simple part molds, but may not be suitable to form complex geometric part shapes because the expanding material may not enter or fill all cavities of the mold. There are also limitations in the foamed material made in this manner due to the inherent material and process limitations.
An injection molding process offers advantages over the steam chest molding and pouring processes. A broader and therefore less expensive range of resin materials can be used with the injection molding process and a more complex part geometry can be obtained, including the use of apertures and ribs to reduce material thickness and vary part stiffness. Several drawbacks exist, however, for current injection molding processes. When reduced density foam parts are required, a gas or a blowing agent is injected with the polymeric material. If the part density is to be reduced by 50% or more, injection times of less than one second are commonly required. To achieve these injection times, a very large molding machine of approximately 3000 to 4000 tons capacity is required to inject approximately 3 to 4 pounds of material into a mold. In addition to the expense of a large capacity molding machine, the shot size is very small relative to the molding machine capacity, resulting in excess wasted material in the injection barrel between shots, as well as degradation with respect to its foaming capability. Further, a large capacity hydraulic system is required to operate the large capacity molding machine, resulting in still further increased costs per part. Still further, the screw must accelerate and decelerate very rapidly, and as the screw mass increases, the difficulty in obtaining accurate shot volumes increases.